This site shows several S6 models and lists only some with LTE-A and it looks like neither the AT&T, Verizon or Sprint phone is LTE-A.
check this out but you will need to add the header because I cannot post links yet
pdadb.net/index.php?m=specs&id=7836&c=samsung_sm-g9250_galaxy_s6_...
If this is correct we will need what is called a zero model to get LTE-A speeds???
Maybe LTE-A will come out later in the year for the US Carriers?